Hi Jakub,
FontAwesome has specific prefixes for class names right? You can try to use advanced search on these prefixes in order to get a list of all places where you used the Font Awesome classes (for example search on "fas" or "fa-".
If you need to replace all these occurances it might be wise to add a specific class to the elements without any CSS declaration assigned (for example: "to-replace"), and remove that once you replaced it
I am also missing some searching methods in Studio. Sometimes it is possible to duplicate the project, delete the unwanted part like the module and just see what errors pop up and what stopped working. It is a rough method, yes. but sometimes it quickly points to the places we are searching for.